Welcome. I'll just copy-paste this because I'm tired of explaining it every single day, in some form or another:

"PDF is the worst possible format to convert FROM. It was designed as an output format. This subject has been beaten to death around here because a lot of PDFs aren't tagged PDFs - meaning that letters (and a lot of times small groups of letters) resemble something like floating objects on a blank paper, each with their own coordinates and extra baggage. So it's very difficult to get a 1:1 conversion. A lot of formatting will be lost, some will get interpreted wrong, etc..."

Here's what you could do:

Run the PDF through ABBYY FineReader 11, proofread it there (meaning you read the book in FineReader), export as .odt or .rtf, do the layout in "Atlantis Word Processor" or "OpenOffice with the Writer2ePub" as these two can export straight to ePub. Then proofread the final product again, preferably on an e-reader or at least in ADE (Adobe Digital Editions) given that most e-reading devices are ADE-powered. Another route would be to use Word 2010, export as Filtered HTML, import the HTML file into Sigil, manually tweak the code but you should know that Word adds a lot of garbage and even the most up-to-date version can't produce a 1:1 HTML file. It's unbelievable. You'd think Microsoft would have the resources to pull it off, but noooo... There's a $99 program called "Word Cleaner" that I have yet to try. For $99 it better do dishes and clean my kitchen too.

If you only need a few certain glyphs you could embed fonts and define them in the style.css file. Otherwise stick to PNG images for the music sheets or whatever.